There has long been a need in industries that handle fluid materials for devices and systems that can measure or estimate properties and conditions of the fluids being handled. Such information can provide a number of benefits including: safer handling of materials, better product quality, lower costs of production, and increased efficiency. There are many properties that can be measured or estimated using such characterization devices. Some commonly measured properties are temperature, pressure, fluid flow rate, density, viscosity, and acidity.
Physical property measurement devices and techniques are used in many industries including, for example, plastics manufacture, petroleum refining, pharmaceuticals manufacture, and food processing, which is a preferred application of the present invention. In food processing, it is very common to process fluid foods and to have a need to determine or estimate the physical properties of the fluid. Some foods processed in fluid form include milk, ketchup, juices, syrups, chocolate and other confectionary, etc. In the processing of such foods, any number of physical properties of the food can be of interest including those properties mentioned above. However, particular properties that are of special concern to the present inventors are solids content and state of dissolution of added solid ingredients or precipitation of solids from the melt. A primary reason for monitoring such properties is to ensure product quality and consistency.
Methods for determining fluid properties can be classified into two types: off-line and on-line procedures. With off-line techniques a sample is typically removed from the system and the desired information is extracted from the sample using a separate device, which is often at a separate location. Off-line methods are generally more labor intensive and take longer to get results. On-line methods are integrated into the processing system itself and generally do not disturb the flow of the process or require removal of a sample. Further, on-line methods typically can provide frequent or even continuous monitoring of the desired property in real time. Thus, on-line techniques are usually preferred over their off-line counterparts and on-line processing is a preferred application of the present invention.
Process control is fairly common in fluid processing and methods and devices to measure many fluid properties have been created. The most commonly used process control systems involve measurement of pressure and temperature but others are known. For example, various on-line techniques have been used to measure solution concentration. Perhaps the most widely used (to measure solution concentration) is the refractive index determination, but a disadvantage of this method is that it requires glass or other transparent material to be brought into contact with the food item. This can raise the complexity and cost of designing and manufacturing the system. Another technique uses conductivity (or other electrical property) measurements to determine the concentration of some solutions.
Other technologies known to be used to characterize food include infrared spectroscopy and nuclear magnetic resonance. These are often used for rapid and nondestructive characterization of foods. Both methods can determine the composition of foods but both are fairly expensive and are not always readily applicable online.
Another technology that has been variously applied to measure fluid properties is ultrasound. Ultrasound consists of high frequency (>20 kHz) sound waves that propagate in materials as small deformations in their structure. Because of this, the capacity of a material to support an acoustic wave as measured as the velocity, attenuation or impedance, is sensitive to both composition and microstructure. There are generally two types of ultrasound waves that can be produced: shear waves and longitudinal waves. In shear waves the deformations are normal to the direction of propagation while in longitudinal waves the deformations are parallel to the direction of propagation. The present invention is primarily concerned with longitudinal waves.
The two general types of ultrasound measurement techniques available are transmittance and ultrasonic reflectance. Ultrasonic transmittance gains useful information about a fluid or other material by measuring how ultrasonic waves are effected as they travel through the material. In ultrasonic reflectance methods, useful information is gained by measuring how ultrasonic waves are effected when the are reflected off of a material to be investigated. Information is gained from the change in signal amplitude and phase following reflection and both parameters may depend usefully on the frequency of the sound. Such changes can then be used to calculate values for various physical properties of the material that are known to correlate linearly or non-linearly with such changes. A potential disadvantage of ultrasonic transmittance methods is that measurement requires the propagation of the sound across a known and fixed distance. This is easy to achieve in a laboratory instrument but it is often a problem in an on-line application where existing equipment may have to be replaced or extensively modified to allow the fitting of the transducers. Reflectance measurements enjoy the key advantage of requiring no modifications of existing equipment.
Ultrasound technologies have many desirable characteristics that make them useful as an on-line sensor (Povey M. “Ultrasonic determination of the properties of food dispersions” Sem Food Anal 4 (2):95-111, 1999). The equipment is relatively inexpensive and robust, the measurements are simple and easily automated, and the sonic beam can pass through container walls and opaque fluids. However, despite this recognition of the benefits of using ultrasound techniques for measuring certain properties of foods, on-line ultrasonic measurements are not widely used for measuring physical properties of food solutions. A possible reason is the disadvantages of transmittance techniques which require that the ultrasound waves must travel a precise and known distance through the sample.
Some particular patents directed at using ultrasound to measure fluid properties in industrial processes in general include U.S. Pat. No. 5,365,778 to Sheen et al. (“Sheen”), U.S. Pat. No. 5,467,321 to Baumoel, U.S. Pat. No. 5,686,661 to Singh et al. (“Singh”), U.S. Pat. No. 6,082,181 to Greenwood (“Greenwood '181”), U.S. Pat. No. 5,708,191 to Greenwood (“Greeenwood '191”) and U.S. Pat. No. 6,227,040 to Hasting et al. (“Hastings”). These patents are hereby incorporated by reference.
The Sheen patent discloses an ultrasonic viscometer and method for measuring fluid viscosity. In this method, ultrasonic shear and longitudinal waves are generated and coupled to the fluid. Ultrasonic shear and longitudinal wave reflections are then detected. From the reflected longitudinal waves, phase velocity of the fluid is determined and from the shear waves viscosity of the fluid is determined. This patent also discloses a self-calibration technique. The primary application of the Sheen technology is for determining viscosity of coal slurry processes.
The Baumoel patent teaches an insertion type ultrasonic transducer assembly adapted to be mounted to a pipe for determining ultrasonic energy transit time through fluid in the pipe. This patent differs from the present invention in that it is concerned only with transmittance of ultrasound shear waves through the fluid whereas the present invention is limited to reflectance of preferably longitudinal waves. Further, the Baumoel patent discusses a common use of ultrasound for determining fluid flow rates and does not address the use of ultrasound in fluid food processing.
The Singh patent discloses a laser enhanced ultrasound method and device for remotely measuring the viscosity of molten materials such as melt glass, melt alloys and the like during processing of the materials. Again this process uses shear waves and is concerned with transmittance of ultrasound through the fluid unlike the present invention. In addition, this patent does not discuss food solutions or making the type of physical property measurements addressed by the present invention.
The Greenwood patents disclose ultrasonic fluid densitometers. The invention includes a wedge having at least two transducers for transmitting and receiving ultrasonic signals internally reflected within the material wedge. It is taught that the wedge should have an acoustic impedance close to that of the fluid being tested.
The Hasting patent discloses an apparatus for determining the viscosity of a fluid in a container such as a tank or pipe. This patent is concerned with ultrasonic transmittance through the fluid.
Despite the broad interest in developing ultrasonic measurement techniques for fluid properties, as indicated by the variety of patents and examples given above, there continues to exist a need for improved techniques and devices. There especially exists a need for improved devices in the food processing industry and particularly for on-line methods. These are the primary needs addressed by the present invention.
Accordingly, it is an object of the present invention to provide an ultrasound method and device to measure certain physical properties of fluids.
It is an additional object of the present invention to provide an ultrasound device that is self calibrating.
Further, it is an object of the present invention to provide an on-line sensor for measuring physical properties of food solutions.